The automotive industry has quality standards that need to be respected when it comes to parts manufacturing. Because of high production rates, automating inspection of these manufactured parts is now necessary. Employing a number of physical principles, numerous efforts have been made through the years to develop systems that are suitable for the automated in-line inspection of automotive glass.
The requirement for simultaneous detection and identifying of a large variety of intrinsic float glass defects and manufacturing/processing defects has applied strong limitations on the design of inspection systems, narrowing down the choice of effective solutions suitable for the above-mentioned purpose.
U.S. Pat. No. 6,437,357 to Weiss et al. is schematically detailed in FIG. 1 and represents the conventional implementation of a flat automotive glass inspection system 10, comprising transmissive directional coherent Bright Field (BF) and transmissive coherent Dark Field (DF) channels. As a matter of cost saving, both the BF and the DF channel illuminators are installed in a same compartment and employ the same collimating lens 11. Two identical DF laser modules 12 are used as light sources for the DF channel and two identical BF laser modules 14 are used as light sources for BF channel. The collimating lens 11 is made of two pairs of identical cylindrical lenses 16. Both BF laser modules 14 are installed at the respective focal points of both pairs of cylindrical lenses 16. As a result, divergent sheets of light, generated by BF laser modules 14 are converted into parallel, or collimated, BF sheets of light 18. Both DF laser modules 12 are installed in a symmetric way further away from the focal plane of both pairs of cylindrical lens 16. Consequently, divergent sheets of light, generated by DF laser modules 12 are converted into convergent DF sheets of light 20. In addition, DF laser modules 12 are slightly inclined relatively to the BF laser modules 14, so that there is a small angle between the emerging BF and DF sheets of light 18, 20. The BF sheets of light 18 and the DF sheets of light 20 pass through a flat glass sample 22, moving along a production conveyor. In FIG. 1, the conveyor would move the glass sample outwardly from the plane of FIG. 1.
The BF sheets of light 18 reach a semitransparent screen 26, attached onto the surface of a Fresnel lens 28. A BF camera 30 with attached BF objective lens 32 is focused onto the semitransparent screen 26. In the absence of defects, glass paintwork or grinded edge, a photosensitive element of the BF camera 30 is illuminated, hence the Bright Field channel name. Any significant departure from stationary illumination level is analyzed by an image acquisition/processing system 34 connected to the BF camera 30. Based on this analysis, the glass inspection system 10 generates a BF defect map for the inspected glass sample 22.
The converging DF sheets of light 20, propagating under an angle to the BF sheets of light 18, pass beside semitransparent screen 26 and Fresnel lens 28 to be absorbed by a spatial filter 36, attached to a DF objective lens 38. The DF objective lens 38 is attached to the DF camera 40. The spatial filter 36 absorbs direct DF sheets of lights 20. Therefore, the photosensitive element of the DF camera 40 is normally not illuminated, hence the Dark Field channel name. If the glass sample 22 has a defect that scatters or diffracts the DF sheets of lights 20 so that they pass beside the spatial filter 36, then they reach the DF objective lens 38, which focuses them onto the photosensitive element of the DF camera 40. Any significant departures from dark level are analyzed by the image acquisition/processing system 34, connected to the DF camera 40. Based on this analysis, the glass inspection system 10 generates the DF defect map for the inspected glass sample 22.
To decrease the dimensions of the glass inspection system 10 and to prevent broken glass drop onto expensive BF and DF objective lenses 32, 38, BF and DF sheets of lights 18, 20 are routed through fold mirrors 44.
U.S. Pat. No. 6,501,546 to Weiss describes an edge inspection module used to inspect the edge quality of flat automotive glass. Four of these modules generate collimated laser light beams, which propagate in the glass conveyor plane. The light beams illuminate the edges of the glass sample that moves along the conveyor. The module has its own built-in imaging system that acquires the light reflected/scattered by the ground edge of the glass. If the edge is free of defects, the intensity distribution of the scattered light across the edge is flat. Any significant departure from this flat intensity distribution is analyzed by an image acquisition/processing system connected to the edge inspection module. Based on this analysis, the glass inspection system generates the edge defect map for the inspected glass sample.
Long-term experience in exploiting both of these glass inspection systems has revealed several serious problems.
First, the use of lasers as light sources for the BF channels has shown that light intensity, passed through the glass samples, is modulated even in the absence of any glass defect. It manifests itself in a way that glass images, acquired with the BF channels, are disfigured with irregular strips of higher and lower light intensity. These intensity variations are known as “draw lines”. This effect has been observed for all glass samples inspected so far, with all glass inspection systems built in correspondence with the conventional art described above. The most pronounced “draw lines” magnitude observed reached 14% of the stationary BF illumination level for clear glass samples with an average light transmissivity of 90-92%. This modulation of the BF light intensity effectively decreases the signal-to-noise ratio (SNR) and degrades the detection capability for a broad variety of defects.
In addition, multiple cases were reported where the glass inspection system falsely reported as defective clear glass samples because the intensity of the BF light inside the glass reached the light intensity outside the glass sample. This happened when the magnitude of the light modulation, combined with BF camera noise, reached or exceeded the intensity of the BF light outside the glass sample. In these cases the glass samples were rejected even though they were not defective.
After investigation, it was discovered that these false defect detections arise because laser light is coherent with a coherence length that is larger than the thickness of the inspected glass samples. As a result, the glass transmission coefficient for coherent light depends on the glass sample thickness even though there is no noticeable absorption inside the glass, such as with clear glass. As soon as the glass sample thickness changes by approximately 0.1 μm, the transmitted light intensity may experience a change of more than 10% depending on the laser coherence length, glass sample thickness, glass complex refractive index and laser light wavelength. Taking into account that the tolerance on automotive glass thickness is approximately 50 μm, the BF images of all glass samples were disfigured to a different extent because of the phenomenon described above.
The semitransparent screen 26 smoothes a strong light interference pattern which occurs when the collimated sheet of coherent light 18 passes through the Fresnel lens 28. However, the point spread function (PSF) width of the resulting BF channel ends up being at least twice as wide as the objective lens 32 PSF. This limits the capability of the BF channel to detect both small defects and defects with low optical density, thereby severely degrading the optical performance of the BF channel.
Furthermore, it has been shown that the combination into one housing of the BF channel, DF channel and edge modules is problematic. Indeed, the light generated by the edge modules and scattered by the glass sample edges, penetrates into the BF and DF cameras and creates overexposed BF and DF image fragments. These overexposed fragments usually are reported as large defects, and glass specimens are rejected even though they are defect-free. If the overexposed area is too large, the system fails to properly inspect it because of a lack of computer memory. In this case, defective glass samples pass the inspection as defect-free even though critical defects may really be present in the overexposed area.
Moreover, an attempt to cut the glass inspection system cost by combining collimating lenses for both the BF and DF channels has created another problem. It has been noticed that both the DF and BF channels have a blind zone in the center of the field of view because of both the imperfections at the junction of the collimating lenses and because of a difficulty with the alignment of the BF and DF beams. The blind zone width ranged from 0.6 to 1.5 mm out of typical 450 mm field of view. Defects traveling through this blind zone, or in close proximity to this zone, were not reported or were reported in a wrong way.
Additionally, the prior art DF channels proved to be incapable to inspect curved automotive glass products such as sidelights, windshields and backlights. It was discovered that, due to convergent the DF light sheet, a DF light source had to be positioned close to the inspected glass sample. Otherwise, the DF channel field of view rapidly decreased. Consequently, light scattering anomalies either inside or on the surface (such as dirt or dust) of the collimating lenses became visible. Light, scattered or diffracted by these anomalies, creates a spiky noise in the background of the DF camera, even in the absence of the inspected glass sample in the DF camera field of view. Although a background correction software algorithm was implemented to correct this problem, it still failed to solve the problem mostly because the scattered light refraction inside curved glass samples is unpredictable.
Glass inspection systems, built in accordance with the prior art, proved to be incapable to inspect dark tinted glass with a transmissivity down to 5% at a conveyor speed up to 800 mm/sec, which are typical automotive glass industry requirements nowadays. Light intensity reaching the cameras and cameras sensitivity proved to be insufficient to provide high-speed inspection for dark tinted glass.
Another problem arises with the inspection of automotive glass products that are partially coated with optically opaque paintwork on their surface. Nowadays, all finished windshields and backlights have such paintwork and final automotive glass inspection requires detecting defects above this paintwork.
The prior art glass inspection systems described above employ transmissive BF and DF techniques (TBF and TDF). These techniques are not adequate for automotive glass inspection above paintwork since light can not be transmitted through the paintwork.
In order to inspect curved automotive glass partially coated with paintwork, a Reflective Dark Field (RDF) inspection system was proposed by Rudert et al. in their PCT patent application published under the No. WO2005/116616. The glass inspection system proposed in this patent application is schematically described in FIG. 2.
Glass inspection system 46 was designed to detect scratches and scuff (abrasion) on a surface 48 of a glass sample such as a windshield 50. The glass inspection system 46 is equipped with an illumination unit 52, which comprises at least one light strip 54. Light, generated by lamps 56 and collimated by optics 58, passes through diffuser 60 and illuminates the surface 48 at an angle 62 relatively to the surface normal 64. A strip of light illuminates an area 66 on the windshield 50, which is imaged by a recording unit 68. A “light box” 70 is installed under the recording unit 68. Flaps 72 are used to shut down the light beams mechanically, because of the inertia of the lamps 56.
The main disadvantage of this system is that it is efficient only for defects with uniform angular scattering/diffraction characteristic. However, this is not the case for many defects like scratches satisfying to standard ASTM F-428-3, -4, -5 for example. During inspection, if the scratch happens to be oriented perpendicularly to the light strips orientation, the glass inspection system will miss them. On the contrary, scratches smaller than ASTM F-428-3 might be detected if their orientation during inspection is parallel to the light strips.
Therefore, there is a clear need for an improved glass inspection system.